The invention relates to circuit fabrication. More particularly, the invention involves the use of a particle filled adhesive coating in circuit fabrication.
Printed circuit boards (PCBs) typically are used as a replacement for wiring to connect electrical components of an intricate electrical system. PCBs save space, weight and labor, and increase reliability of the circuit as compared with round wire connections. Instead of metal round wire, the conventional PCB includes a metal foil layer (e.g., a copper foil layer) which is adhered to a base (e.g., a fiberglass/epoxy board). Once adhered to the base, the foil layer is then imaged and etched into a desired pattern of conductive lines to form a circuit. Foil layers may be adhered to both sides of the base to produce a double-sided circuit.
A particular type of printed circuitry, often called flexible circuits, or "flex," has the same basic structure as conventional hard PCBs, or "hardboards." Flexible printed circuits typically offer many advantages over standard hardboard printed circuits and round wire technology. For example, due to its flexibility, connection of a flexible circuit to a read/write head of a hard disk drive permits cycling of the head across the disk drive many millions of times.
The flexible printed circuit is formed from at least one conductive foil layer (e.g., a copper foil layer) that is adhered to a flexible base film. This base film typically is an electrically nonconductive, or dielectric, film substrate, such as polyimide film or a polyester film (e.g., PEN film or PET film). Similar to the PCB, once adhered to the base film, the foil typically is then imaged and etched into a desired pattern to form a circuit. The foil may be adhered to both sides of the base film to produce a double-sided flexible circuit. Adhesives (e.g., polyimides, epoxies and acrylics) may be used to bond the foil layer to the film substrate. However, the foil layer may also be adhered to the film substrate without adhesives by using, for example, plating or vapor deposition.
Although the base film provides surfaces for a double-sided circuit, many complex circuit designs require more than two layers for electrical connection requirements. Therefore, quite often the flexible circuit requires additional copper layers to yield a "multi-layer" circuit. The inner double-sided circuit is often called the core of the flexible circuit.
Referring to FIG. 1, as an example, to form a multi-layer circuit 10, metal foil layers 12 (only one foil layer 12 is shown in FIG. 1) are adhered to, or laminated, to both sides of a dielectric film substrate 14 to create an inner core 16. The foil layers 12 are then imaged and etched to form conductor lines 18, as shown in FIG. 2, (only one conductor line 18 is shown in FIG. 2). Thus, the inner core 16 provides up to two etched foil layers of conductor lines.
To form additional etched layers, additional outer foil layers 20 may be laminated to inner core 16 on top of the existing etched foil layers (i.e., on top of existing conductor lines 18). Outer foil layers 20 may then be etched to create additional conductor lines.
Lamination of the foil layers typically includes the use of high temperature and high pressure. As a result, the adhesive that bonds adjacent layers together tends to flow away during lamination. In this manner, conductor lines on different layers may contact each other. To prevent this from occurring, dielectric films 22 are placed between adjacent foil layers.
Thus, multi-layer flexible circuits have traditionally used dielectric films to separate adjacent etched foil layers. As a result, typically, the overall thickness of the flexible circuit is limited by the thickness of these layers.